Ceramic - Pottery Dictionary

by Susan Mussi

KILN - History

ca: FORN - Historia

es: HORNO - Historia

Primitive man knew that leaving clay in the sun hardened it and when he learnt how to make fire he realized that putting clay for several hours on a bonfire, with the heat all round, made it become even harder and stronger and that many pieces could be fired together. As man evolved over thousands of years, so did the method of firing. The next step came with the realization that the earth did not burn and held in the heat. Pits were dug into the ground and layers of flammable organic material such as wood and straw were laid on the floor. The sun-dried pots would be stacked on top, then more organic material round the sides and finally broken pieces of fired pottery over the top. When fired, the earth walls and the broken pottery held in the heat so firing became quicker and reached a higher temperature.

The next stage of progress after pit firing was to an updraft kiln and it is believed that this started in Egypt over 7000 years ago. Man realized that if the earth and the walls of a pit could hold the heat then a kiln could be built up with stones, broken pottery and mud-clay. The first updraft kiln was oval in shape, like a U upside down, with an opening at the bottom to use as a stoke hole. The roof was a half circle and completely sealed off for firing and had to be broken to unload. This method spread throughout the Mediterranean countries, Greece, Rome, Crete and the Middle East.

Great improvements came in the next stage; the kiln was separated into two sections by a floor. The bottom was for the wood for heating, it had open spaces and doors so wood could be continually added. The ware being fired was on the top floor and had a chimney so the gases could escape during the early hours of firing. Both the chimney and the stoke holes had separate blocks of bricks to cover them during the firing; this helped to control and increase the heat.

The downfall of the Roman and the growth of the Arab Empire brought great changes in the understanding of ceramics, the use of glazes and colors made from valuable metals. There came the understanding that clay firing and glaze firing had to be separate as gases from the clay damaged the colors.

In Spain, the kilns used for firing were known as Arab Kilns and they created the heat by wood-firing. They were large brick square structures with two floors. The bottom floor of the kiln was for the wood-fire and the top for the objects to be fired. The firing time depended on the size of the kiln and could take up to 24 hours. Firing started with a low fire, gradually heating up until it reached the required temperature, this point being judged by looking through the pin-holes to see the color of the heat and slipping out test pieces to see the evolution of a colored glaze. The timing with clay was not so important, as slightly over firing does not affect it, but with colors, over firing can completely destroy all the work. The firing process demanded men to work non-stop, filling the kiln with wood, checking that the temperature was continuously increasing at the same degree throughout the kiln. When the kiln reached the correct temperature it was left to cool, opened, and each piece of bisque was taken out and checked. It was with this method of firing that all the beautiful Luster was produced.

The energy for kilns during many centuries was the heat from burning wood, but now there are many different types of kilns, makes, sizes, designs, firing and the energy for heating can be electric, gas, wood, coal and oil. They are made for use with an automatic temperature controller or, if electric, with cones. Some are known by their method of glazing like Raku and Salt and the understanding of kilns, with their many differences, can become a specialty in itself.